1. Field of the Invention
The present invention relates to a polishing apparatus in general and more particularly to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish with an abrasive cloth.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused on by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surface of semiconductor wafers is to polish with a polishing apparatus.
Such a polishing apparatus has a turntable and a top ring that rotate at respective individual speeds. An abrasive cloth is attached to the upper surface of the turntable. A workpiece such as a semiconductor wafer to be polished is placed on the abrasive cloth and clamped between the top ring and the turntable. During the operation, the top ring exerts a constant pressure to the turntable, and a slurry-like abrasive material is sprayed from a nozzle over the abrasive cloth. The abrasive material enters the gap between the abrasive cloth and the workpiece. The surface of the workpiece held against the abrasive cloth is therefore polished while the top ring and the turntable are rotating.
As the polishing process progresses, the abrasive cloth is clogged with abrasive grains contained in the abrasive material. At certain intervals, therefore, the abrasive cloth should be dressed to make itself ready for reuse by removing the clogging abrasive grains. For this purpose, the polishing apparatus is usually equipped with a dressing device.
FIGS. 7(a) and 7(b) of the accompanying drawings show a conventional dressing device for dressing an abrasive cloth. As shown in FIGS. 7(a) and 7(b), the dressing device has a brush 32 attached to an arm 31. To dress an abrasive cloth 34 mounted on an upper surface 33a of a turntable 33; the turntable 33 is rotated about its own axis, and the lower end of the brush 32 is held against the abrasive cloth 34. At the same time, a cleaning solution W such as pure water is ejected from a nozzle 35 onto the abrasive cloth 34.
The conventional dressing device is however disadvantageous in that it fails to clean the entire surface of the abrasive cloth 34 uniformly and cannot fully remove the abrasive grains which have embedded in the abrasive cloth This is because, as shown in FIG. 9(a), the abrasive cloth 34 is swept only in one direction depending on the rotational direction of the turntable 33, and hence the abrasive grains are removed from the abrasive cloth 34 only in one direction. Consequently, even though the abrasive cloth 34 is dressed by the dressing device, the abrasive cloth 34 has a relatively short service life, and must frequently be replaced.
Japanese laid-open utility model publication No. 63-97454 discloses another conventional dressing device. As shown in FIGS. 8(a) and 8(b) of the accompanying drawings, the conventional dressing device has a radial brush 42 mounted on a rotatable shaft 41. To dress an abrasive cloth 34 attached to an upper surface 33a of a turntable 33, the turntable 33 is rotated about its own axis, and the brush 42 is rotated by the shaft 41 about the axis of the shaft 41. While the brush 42 is being held in contact with the abrasive cloth 34, a cleaning solution W such as pure water is ejected from a nozzle 35 onto the abrasive cloth 34.
The dressing device shown in FIGS. 8(a) and 8(b) has a similar problem in that it fails to achieve uniform cleaning of the entire surface of the abrasive cloth 34 and full removal of the abrasive grains which have stuck to the abrasive cloth 34. This is also because, as shown in FIG. 9(b), the abrasive cloth 34 is napped only in one direction depending on rotational directions of the turntable 33 and the brushes 42, and hence the abrasive grains are removed from the abrasive cloth 34 only in one direction.
Furthermore, as shown in FIGS. 7(a), 7(b) and 8(a), 8(b), when the turntable 33 is stopped at the end of a polishing process, the abrasive cloth 34 quickly dries because the solution in the slurry-like abrasive material that has seeped in the abrasive cloth 34 evaporates, Repeated drying cycles make the abrasive cloth 34 relatively short in service life.